A fuse is a protective device for electrical circuits which has a fusible element that melts and opens to interrupt the circuit when subjected to excessive currents. The melting occurs, in large part, due to I2R heating of the fusible element. For many types of fuses, melting at relatively high currents (i.e., currents that produce melting in less than about 1 second) typically occurs at one or more reduced cross-sectional areas of the fusible element, so designed as to control the melting time versus current characteristic of the fuse (melting time-current characteristic or TCC). The TCC is an important characteristic of the fuse that enables it to provide appropriate system protection and coordination with other devices. Favorable ratios of rated continuous current to short time melting current frequently require the use of relatively high melting point materials for the fusible element, such as silver and copper. At longer melting times, a common practice is to employ an additional means to initiate melting and arcing, using a lower melting point material. This is done in order to prevent excessive fuse component temperatures before circuit interruption, and to provide suitable graphical curves of TCC for typical applications.
The low melting point material is typically implemented in one of three ways. The first, termed the “M” effect (after its discoverer, Metcalf), attaches the low melting point material to the high melting point element in such a way as to cause the high melting point material to dissolve into the lower melting point material when the latter becomes molten. Thus, after a period of time, the element is severed and an arc is initiated at the melted open point.
A second method, disclosed in U.S. Pat. No. 5,604,474 to Leach and Bennett, employs a tin low melting point material as a “bridge” between high melting point elements, wherein an arc is initiated when the tin melts. This method does not suffer from the potential deterioration problems inherent in the first method, but is harder to manufacture.
A third method known in the art employs a spring loaded joint, termed a “mouse trap,” made between high melting point components using a low melting point solder. When the melting point of the solder is reached, the spring causes separation of the contacts thereby producing an arc. This method allows considerable flexibility in the design of the fuse's TCC, since the mass of the components and melting point of the solder can be used to control the melting characteristics, frequently allowing superior surge withstand for the fuse. However, in order to allow the physical movement that initiates circuit interruption, the joint has to be surrounded by a fluid (usually air) rather than by sand, which is the preferred medium to surround a fuse element because it gives improved interrupting capability to the fuse.
Whatever method is used to initiate arcing at longer melting times, a common requirement for such fuses is the provision of some form of indication that the fuse has indeed operated. This makes finding the “blown” fuse much easier.
The most common method of indication is to run a small conductive wire in parallel with the main element(s). When the main element melts, system voltage causes current to flow through the indicator wire and to melt it. The current quickly switches back to the main elements, which then arc and interrupt the overcurrent. The melting of the indicator wire provides indication through a variety of means. Most commonly, the indicator wire is arranged to release a spring loaded pin, or ignite a small explosive charge to move a striker, when the indicator wire melts. Obviously a minimum voltage, sufficient to drive enough current through the indicator wire to cause it to melt, is necessary for this indication method to work. Typically this requires at least 5-10 volts.
Another means of indication has been to connect, in parallel with the fuse, a circuit containing a light emitting device, such as a neon, LED or lamp. Again, system voltage across the indication circuit after the fuse has operated is necessary for this method to work.
For most conventional low voltage fuse applications, the techniques described above are sufficient to successfully interrupt fault currents, provide indication and then withstand system voltage for very long periods of time. However, there are some applications for which these conventional low voltage fuses are not suitable. For example, in some applications, where step-down transformers are used to supply a low voltage distribution network consisting of many parallel conductors fed from many transformers at different points in the system, it is common practice to fuse individual cables to prevent them from being overloaded. These cables typically use conventional current-limiting fuses or fusible limiters, designed to open when excessive current flows. However, it is possible in such applications for little or no voltage to appear across the limiter after the circuit is opened (only the IR drop in the parallel cables will appear across the open point), because an overloaded cable can often have other cables connected in parallel.
This leads to two potential problems. The first is that with little voltage across the limiter when melting occurs, there is little arcing. This can lead to a relatively high resistance open point, sufficient to prevent current flow in the overloaded cable, but which is not high enough to enable conventional fault finding equipment to be effective at finding the open point, as compared with a fuse which arcs and which would normally have a resistance of many millions of ohms. This delays the finding and replacing of the operated device.
Visual indication of the operated limiter would obviously be desirable to speed up the process. However, the second problem is that the lack of recovery voltage also prevents conventional indicators from working.
Accordingly, there is a need for a fuse capable of interrupting current effectively for conditions where the voltage can vary from rated voltage, down to very low values (possibly as low as 1 V) whilst providing indication of its operation in a manner that allows visual indication at the fuse, together with remote indication if desired. Such indication is needed with whatever current causes the fuse to melt open, and with what ever degree of arcing that occurs.